The fundamental objective of this proposal is to elucidate the mechanism of reversion of Rous sarcoma virus (RSV)-transformed field vole cells. From our preliminary analysis of RSV-transformed revertant vole cells it appears that the loss of the transformed phenotype is not due to either a loss of the viral genome or regulation of transcription in these cell types since the RSV transforming gene product (pp60src) can be identified in revertant subclones. Moreover a biologically active RSV genome can be resuced from revertant subclones by either cell fusion or DNA trasfection. Therefore reversion in RSV-infected vole cells must be due to either 1) a difference in the size and/or structure of src-specific mRNA; 2) incomplete post-translational modification of pp60src or, 3) a defective host cell gene product with which pp60src must interact to facilitate transformation. In an effort to distinguish between these possibilities we propose to analyze the structure and function of pp60src in transformed and morphologically reverted vole cells as well as its specific mRNA species. Since we have obtained two classes of morphological revertant subclones containing pp60src (i.e. one class that is tumorigenic and another that is not) we should be able to distinguish between the mechanism by which RSV converts the cell to one that causes tumors in animals and the mechanism by which the virus changes the morphological properties of the cell in vitro. The methods proposed in this analysis include immunoprecipitation/PAGE of pp60src, peptide mapping, subcellular fractionation, affinity chromatography, two-dimensional gel electrophoresis of proteins and RNase T1-oligonucleotides, DNA and RNA sequencing, and immuno-electron microscopy.